How can I make a warning sign appear when an enemy is behind my character in game engine?
The near sensor is checking if an object is near.

You requirement is to detect if the enemy is behind the character. Either you use a radar (looking behind the character), or you use the near and check if the detected objects are behind the character.

Nevertheless the near sensor should detect objects already. Are you sure the enemies are in range and detectable (dynamic, or rigid body)?